1. Field of the Invention
The invention relates to a vehicle drive apparatus that drives wheels by using motor torque.
2. Description of the Related Art
Conventionally, there has been known a four-wheel-drive vehicle configured such that main drive wheels, which are either front wheels or rear wheels, are driven by an internal combustion engine (engine), and auxiliary drive wheels, which are the wheels other than the main drive wheels, are drive by an electric motor. A vehicle drive apparatus mounted in the four-wheel-drive vehicle of this kind is described in, for example, Japanese Patent Application Publication No. 2008-185078 (JP 2008-185078 A).
The vehicle drive apparatus described in JP 2008-185078 A includes an electric motor (motor), a speed reducer, a differential, and a connection-disconnection mechanism. The speed reducer reduces the speed of rotation output from the electric motor. The differential distributes the output from the electric motor to right and left wheels after the speed is reduced by the speed reducer. The connection-disconnection mechanism connects (interconnects) one of a pair of side gears, serving as output members of the differential, and the wheel (left rear wheel) to each other, and disconnects the one of the side gears and the wheel from each other.
The connection-disconnection mechanism includes a first spline gear on the side gear side, a second spline gear on the wheel side, and a synchro sleeve movable in the direction of the axle shafts. When the synchro sleeve meshes with both the first spline gear and the second spline gear in the connection-disconnection mechanism, the side gear and the wheel are connected to each other so that torque is allowed to be transmitted therebetween. The connection-disconnection mechanism is configured such that the side gear and the wheel are disconnected from each other when the mesh of the synchro sleeve with the first and second spline gears is cancelled.
However, in the vehicle drive apparatus described in JP 2008-185078 A, when the synchro sleeve is moved while torque is being transmitted between the first spline gear and the second spline gear, it is necessary to push the synchro sleeve against the force of friction between internal splines of the synchro sleeve and the first and second spline gears. Because the force of friction changes on the basis of the torque output from the electric motor, the capacity of a mechanism (hydraulic circuit) that moves the synchro sleeve needs to be large, taking into account the maximum output torque of the electric motor. This becomes a factor of increases in cost and weight of the apparatus. Furthermore, if the force that pushes the synchro sleeve is not sufficiently large with respect to the force of friction that is generated in accordance with the movement of the synchro sleeve, it is not possible to move the synchro sleeve quickly.